When engine oil becomes contaminated with the by-products of combustion, its value as a lubricant is greatly diminished. The main contaminate in engine oil during normal combustion is carbon. Diesel engines produce large amounts of carbon referred to as soot during combustion, and the measurement of the percentage of soot in the diesel oil gives an indication of when the oil should be changed.
Measurement attempts at DC and low frequency AC (i.e., below about one kHz) fail because the change in conductivity is very small for large changes in the percentages of soot. At high frequencies (i.e., in the MHz range) the AC conductivity changes due to the soot are measurable even at levels of less than one percent soot. The problem with the high frequency AC conductivity measurement is that the sensor, which defines a measurement volume, is a capacitor. Since the sensor has a capacitance associated with its physical shape, there is also a capacitive reactance associated with the sensor. The problem arises because the oil capacitance conductivity is high at these high frequencies, and the conductivity contribution of the soot in oil (small conductive particles mixed with oil), which can be theoretically modeled as a capacitance-resistance network, is relatively low. There are methods that are used in a laboratory that can make the measurement, but the equipment is expensive and the setup must be nearly ideal (very short leads). The use of a network analyzer or vector voltmeter is cost prohibitive. An RF bridge measurement could be used if the resistance and capacitive reactance were near the same values. These drawbacks make such measurements very difficult in the real-world environment of an operating engine.
Accordingly, what is needed in the art is a method, applicable to real-world engine operation environments, for determining contamination of Diesel engine lubrication oil by measuring the AC conductivity thereof at high frequency.